The present invention relates generally to direct current motor control circuitry and more particularly to a solid state motor speed control circuit which varies the motor speed by varying the duty cycle of the voltage applied to the motor.
Basic motor speed control circuits have in the past employed a fixed direct current voltage source with a variable resistance in series with, for example, the motor armature, so that an operator might vary the series resistance thereby also varying the voltage applied to the motor armature and therefore also the motor speed. This series variable resistor arrangement provides adequate speed control, however, particularly at reduced speeds, substantial I.sup.2 R losses occur in the series variable resistance, resulting in substantially reduced efficiency of the system. In installations employing a rechargeable storage battery, such as an electrically powered golf cart or forklift truck, a large fraction of the energy consumed by the system is dissipated as heat from the series control resistor and the elimination of this heat loss would provide for either a substantial increase in the operating time between recharging sessions or a substantial reduction in the size of the storage battery required while maintaining the operating time between battery chargings substantially the same.
In addition to the inefficiency and heat dissipating problems, such series resistance controls are plagued with electro-mechanical contact problems including the service and maintenance problems associated therewith. Motor overload protection circuitry is difficult or expensive to incorporate and the frequent recharging of the energy source shortens the life of that source. Despite these problems, the series control resistor is still commercially employed because of its relatively low initial cost.
Various attempts have been made to avoid the foregoing disadvantages of series resistance control arrangements by, for example, electronically controlling the duty cycle of energy supplied to a motor. Such electronic controls may, for example, employ a silicon controlled rectifier in series with the motor armature and selectively gate that silicon controlled rectifier to conduct during selectable portions only of repeated time intervals. These prior art duty cycle motor controllers have not met with great commercial success, due in part to their relatively high initial cost and lack of overload protection for the control circuit elements.